Roof bolting is used as the primary means of roof support in underground mines using the room and pillar mining method, currently between 85% and 90% of U.S. underground coal mines.
The invention pertains to those roof bolts which are grouted in place, either along their entire length or along a portion of their length. In this application of roof bolting, typically four to six foot deep, one inch diameter holes are drilled in the overlying rock strata. These holes are typically spaced on a four foot square grid. Sausage shaped cartridges are inserted in these holes, often temporarily held in place by plastic wedges or caps. The cartridges contain the components of the grout. The grout is normally separated in two portions which, when mixed together, cause the grout to solidify in a competent mass. In the popular polyester resin grout, one of the two portions of the grout contains primarily a thermoset monomer, which is extended with fillers; the other of the two portions contains a catalyst. The two portions are separated in the cartridge by a longitudinal membrane. In a more recent innovation, the two portions are gypsum powder and water, the water being held in separation from the gypsum by encapsulation in 1/16 inch diameter wax containers, which are dispersed through the gypsum in the correct proportion for solidification after mixing. After the cartridges are placed in a previously drilled hole in the mine roof, subsequent insertion of the reinforcing roof bolt, usually a deformed steel bar, with a bolt head and roof plate at the lower end, ruptures the separating membrane or the wax pellets, as well as the wrapping material which forms the cartridge, thus initiating contact between the two components of the grout and spilling the grout in the annular space between the roof bolt and the rock wall of the hole. The roof bolt is rotated for a short period of time to promote mixing of the two portions of the grout. The rotating action also heats the grout, further promoting a rapid cure. After a brief interval, rotation of the bolt is stopped and the bolt is held in position for a period of sufficient duration, typically somewhere between 20 seconds and a minute, for the grout to cure sufficiently to prevent the bolt from falling out of the roof, if the bolt is no longer held.